This invention generally relates to aircraft area navigation which includes an omnidirectional radio range and bearing transmitter, referred to hereafter as VORTAC, located at various ground stations which can define waypoints along the course of an aircraft flight plan and particularly to aircraft radio navigation systems which control lateral transitions of the aircraft from an inbound course to an outbound course within the limits of controlled airspace around the waypoints.
Many of today's aircraft are equipped with radio navigation systems which include a flight management computer which among other things provides for both vertical and lateral navigation of the aircraft. In the lateral navigation mode the computer functions to navigate and guide the aircraft along a flight plan made up of a series of waypoints connected by courses defining point to point legs of the flight plan. Air traffic control regulations specify the protected airspace width on either side of such legs. The regulations also specify both an enroute zone and terminal zone which further defines the width of the protected airspace. The width of the protected airspace in a terminal zone being much narrower than the width of the protected airspace in the enroute zone. For example, the present regulations specify that the protected airspace width of the enroute zone is four nautical miles, while the protected airspace width of the terminal zone is two nautical miles. In the present invention, for example, the enroute zone width restriction applies to aircraft flying above 18,000 feet of altitude while the terminal zone width restriction applies to aircraft flying at or below 18,000 feet of altitude.
In today's heavy air traffic environment most flight plans contain multiple waypoints between origin and destination. Each of these waypoints defines a flight plan leg. The aircraft must then transition between each leg of the flight plan from the origin to the destination.
The problem at each of these transitions is to keep the aircraft confined within the protected airspace width while at the same time minimizing the aircraft bank angle in the interest of passenger comfort.
In prior lateral course transition systems such as that disclosed in U.S. Pat. No. 3,994,456 assigned to the assignee of the present invention, the transition apparatus first chooses a bank angle command based on the course change at a given waypoint and together with the ground speed of the aircraft calculates a turn radius based on the bank angle command and the aircraft ground speed. An arc having the calculated turn radius is fitted into the waypoint corner defined by the inbound and the outbound legs of that waypoint. The arc is fitted such that the arc is tangent with each of the legs. If the ground speed changes before the aircraft enters the transition arc, the turn radius is recalculated and a new arc is fitted into the waypoint corner.
Although the above described apparatus has performed in an acceptable fashion in the past, the current airspace restrictions around a waypoint are not fully addressed, since the prior apparatus does not provide information about the protected airspace width. Therefore, the prior apparatus does not utilize all of the available airspace which results in aircraft bank angles larger than necessary and the aircraft taking a longer than necessary transition path. In addition, if there is a change in ground speed while the aircraft is in the transition arc the aircraft can overshoot the protected airspace outer boundaries of the outbound leg of the waypoint. All the foregoing disadvantages of the prior apparatus can lead to passenger discomfort, increased fuel usage, and an increase in travel time.
The present invention overcomes the foregoing problems by providing an aircraft transitioning from an inbound leg to an outbound leg of a lateral navigation waypoint with a fixed curved transition path within or along the outer limit of the protected airspace width. The present invention determines in conjunction with the aircraft altitude and the angle of the course change a maximum distance that the aircraft will deviate from either the inbound or outbound leg. This distance is sometimes called the maximum cross track error or MXTK. The maximum cross track error information in conjunction with the course change angle is used to generate a turn radius for a fixed curved transition path to be followed by the aircraft independent of ground speed. The turn radius information together with the aircraft ground speed information is used to generate a bank angle bias command signal. The bank angle bias command is calculated to vary the bank angle of the aircraft so as to keep the aircraft on the fixed curved path throughout the transition from inbound to outbound waypoint legs.
Thus it is appreciated that the above described problems are eliminated in a manner as described hereinafter and that further advantages and details of the invention can be had from the following description and claims taken together with the accompanying drawing.